Daniel Fallon and Nick Carey

Ferguson, St Louis: Violence again erupted in the St Louis area near the site of the police shooting death of an unarmed black teenager, according to local police, despite calls by US President Barack Obama and activists for a measured response.

Tuesday night started with a peaceful protest by a small gathering of well-meaning black and white residents on the main street of Ferguson.

Police had cordoned off West Florrissant Avenue - the scene where multiple businesses were looted on Saturday and Sunday night and where police confronted protesters using rubber bullets and tear gas on Monday night. The Quick Trip convenience store that burnt to the ground on Sunday has become a rallying point for protesters.

Traffic was diverted by police in an effort to minimise the risk of further civil unrest.

A demonstrator throws back a tear gas container after tactical officers worked to break up a group of bystanders on Chambers Road and West Florissant early on Wednesday morning. Photo: AP

However, early on Wednesday, a police officer shot and critically wounded a man who drew a handgun near the site of protests over the death of 18-year-old Michael Brown, a St Louis County Police Department officer said.

Police responded about an hour after midnight to reports of four or five men with shotguns who wore ski masks. They encountered multiple suspects running, one of whom pulled a gun on an officer, who fired at him, the county officer said. The man was taken to an area hospital.

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Shortly after midnight, police fired tear gas into protesters who had confronted a line of officers after a far larger crowd dispersed, St Louis County Police Department spokesman Brian Schellman said.

A photograph in the St Louis Post-Dispatch showed a protester wearing a shirt with an American flag printed on it throwing a tear gas container back at the police. There were other media reports of bottles thrown at police.

Demonstrators protest the killing of teenager Michael Brown outside Greater St Marks Family Church while Browns family along with civil rights leader Reverend Al Sharpton met inside to discuss the killing in St Louis on Monday. Photo: Scott Olson/Getty Images

"I know the events of the past few days have prompted strong passions, but ... I urge everyone in Ferguson, Missouri, and across the country, to remember this young man through reflection and understanding," Mr Obama said in a statement.

Friends and family of Brown held a peaceful church vigil on Tuesday night, after his father pleaded for an end to the violence. Standing with supporters, including the Reverend Al Sharpton, Michael Brown Sr. said he wanted justice for his son but wanted it "the right way."

"I need all of us to come together and do this right, the right way," said Brown Sr, who wore a T-shirt showing his son's baby picture. "No violence."

Several hundred protesters appeared to heed the calls for non-violence late on Tuesday evening, chanting "hands up, don't shoot" and "no justice, no peace" during a tense but ultimately peaceful stand-off with police clad in riot gear and flanked by armoured vehicles near the site of Brown's death.

President Barack Obama has called the shooting death of Michael Brown a tragedy, but urged a measured response from Ferguson residents. Photo: AP

The protesters, some of whom waved signs as the group was led in chants by megaphone, had dwindled to a handful before midnight.

"My message I'm trying to get across is for Mike Brown - he needs justice for what a happened to him," said Iesha Owens, who has spent four days protesting in Ferguson. "And not only just Mike Brown, but Trayvon Martin and anybody else who needs justice needs to be served."

Demonstrators protest the killing of teenager Michael Brown outside Greater St. Marks Family Church in Ferguson on Monday. Photo: Scott Olson/Getty Images

However, with the ongoing protests and media coverage of the situation will help pressure authorities to act on the case, she said. For Ms Owens, justice means the police officer who shot Michael Brown will be prosecuted.

"I believe there will be justice served. I believe the cop will go to jail for killing Mike Brown. I really believe it."

Also on Wednesday, a woman was shot in the head in a drive-by shooting blocks from the area where Brown was killed. Her condition and whether the shooting was related to the protests were unknown, Mr Schellman said.

In a separate incident simmering in California, a vigil was planned after Monday's shooting death of an unarmed 24-year-old black man in Los Angeles, USA Today cited a Los Angeles Police Department spokeswoman as saying.

Controversial shooting

Mr Sharpton, a New York-based civil rights leader, called for peaceful protest in the wake of looting and more than 50 arrests since the shooting.

"To become violent in Michael Brown's name is to betray the gentle giant that he was," Mr Sharpton said of the 193 cm (6-foot, 4-inch) Brown, who had planned to start college this week.

Missouri Governor Jay Nixon told a packed church in North St Louis County on Tuesday evening the community was "reeling from what feels like an old wound that has been torn open afresh."

The activists also were demanding authorities make public the name of the officer involved. The police had said they would do so on Tuesday, but changed the plan, citing fears of retaliation, according to media reports.

Police said Brown was shot in a struggle with a gun in a police car but have not said why he was in the car. At least one shot was fired during the struggle, and then the officer fired more shots before leaving the car, police said.

A witness to the shooting interviewed on local media has said that Brown had been putting his hands up to surrender when he was killed.

The FBI has opened a civil rights investigation into the racially charged case and St Louis County also is investigating.

Demonstrations on Sunday night turned violent, with looting and property damage. Violence broke out again on Monday night as police officers in riot gear, armed with rifles and accompanied by dogs tried to secure the area.

Residents in the low-income, mostly black neighbourhood where Brown was killed say they are often harassed by police. Ferguson Police Chief Tom Jackson said the neighbourhood had a lot of crime but there were no race problems.

Ferguson has seen a stark demographic shift in recent decades, going from all white to mostly black. About two-thirds of the town's 21,000-strong population are black. On a police force of 53, three officers are black.